1. Field of the invention
The present invention relates to an automatic transmission for use in an automotive vehicle, and more particularly to an automatic transmission having an overdrive gear mechanism positioned between a hydrokinetic torque converter and an underdrive mechanism.
2. Description of the prior art
As is well known, automatic transmissions for automotive vehicles automatically change gear reduction ratios in response to vehicle speed and load on the vehicle engine. An automatic transmission in general consists of a torque converter and an underdrive gear mechanism which is coupled to the converter and establishes two or more gear reduction ratios of no less than 1. Recently, a demand for an automatic transmission with overdrive has been arisen particularly from the viewpoint of improving the drive feeling to the operator, improving economy and permitting better emmision control of the engine.
In case an overdrive gear mechanism is added to an automatic transmission having a torque converter and an underdrive gear mechanism, when gear strength and capacity of frictional engaging means in the overdrive gear mechanism are desired to be increased for improving durability and manufacturing cost, it is advantageous to position the overdrive gear mechanism between the torque converter and the underdrive gear mechanism rather than on an output side of the underdrive gear mechanism (rear-side type). In other words, an output side of the overdrive gear mechanism is connected to an input side of the underdrive gear mechanism (front side type). In addition, the automatic transmission is equipped with a torque converter and a hydraulic control device, so that an oil pump is required for delivering a hydraulic pressure thereto. The oil pump should be driven by being coupled to an pump impeller in the torque converter, so that in the case of the front side system, the oil pump is provided within a wall partitioning the torque converter from the overdrive gear mechanism. Furthermore, the overdrive case which encompasses the overdrive gear mechanism therewith partitions the overdrive gear mechanism from the underdrive gear mechanism, with the result that the underdrive gear mechanism is housed in a chamber or space which is independent of the underdrive gear mechanism.
During deceleration, respective elements of the overdrive gear mechanism rotate jointly. However, during overdrive, or accleration which occupies a majority of the operating time, respective elements of the overdrive gear mechanism rotate relative to each other for achieving a reduction gear ratio of less than 1. This relative rotation leads to generation of heat as well as development of friction. Thus, oil should be supplied to cool and lubricate the overdrive gear mechanism. However, as has been described earlier, the overdrive gear mechanism is housed in a closed chamber, so that another new oil pan would be necessary under the overdrive gear mechanism in addition to an existing oil pan, thereby discharging oil which has been used for cooling and lubrication, from the overdrive gear mechanism into the aforesaid another oil pan. The addition of such an oil pan is not desirable from the viewpoint of manufacturing cost and saving in weight. Additionally, utilizing a lower inner surface of the overdrive case as an oil sump by increasing the size of the overdrive case is also undesirable.
It is accordingly an object of the present invention to provide an automatic transmission with overdrive of a front-side type, which allows suitable and effective discharge of oil from the overdrive gear mechanism, after cooling and lubrication to an oil pan, for recirculation from the oil pan through an oil pump, a hydraulic control device, and an underdrive gear mechanism and back to the overdrive gear mechanism.
It is another object of the present invention to provide an automatic transmission with overdrive of the type described, which provides means for discharging oil from the overdrive gear mechanism to reduce the weight of the automatic transmission and allow reduction in the size thereof.